1. MIT AITI 2004 – Lecture 14 Exceptions Handling Errors with Exceptions

2. Attack of the Exception What happens when this method is used to take the average of an array of length zero? Program throws an Exception and fails java.lang.ArithmeticException: / by zero public static int average(int[] a) { int total = 0; for(int i = 0; i < a.length; i++) { total += a[i]; } return total / a.length; }

3. What is an Exception? An error event that disrupts the program flow and may cause a program to fail. Some examples: Performing illegal arithmetic Illegal arguments to methods Accessing an out-of-bounds array element Hardware failures Writing to a read-only file

4. Another Exception Example What is the output of this program? public class ExceptionExample { public static void main(String args[]) { String[] greek = {"Alpha", "Beta"}; System.out.println(greek[2]); } Output: Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 2 at ExceptionExample.main(ExceptionExample.java:4)

5. Exception Message Details What exception class? Which array index is out of bounds? What method throws the exception? What file contains the method? What line of the file throws the exception? Exception message format: [exception class]: [additional description of exception] at [class].[method]([file]:[line number]) Example: java.lang.ArrayIndexOutOfBoundsException: 2 at ExceptionExample.main(ExceptionExample.java:4) ArrayIndexOutOfBoundsException 2 main ExceptionExample.java 4

6. Exception Handling Use a try-catch block to handle exceptions that are thrown try { // code that might throw exception } catch ([Type of Exception] e) { // what to do if exception is thrown }

7. Exception Handling Example public static int average(int[] a) { int total = 0; for(int i = 0; i < a.length; i++) { total += a[i]; } return total / a.length; } public static void printAverage(int[] a) { try { int avg = average(a); System.out.println("the average is: " + avg); } catch (ArithmeticException e) { System.out.println("error calculating average"); }

8. Catching Multiple Exceptions Handle multiple possible exceptions by multiple successive catch blocks try { // code that might throw multiple exception } catch (IOException e) { // handle IOException and all subclasses } catch (ClassNotFoundException e2) { // handle ClassNotFoundException }

9. Exceptions Terminology When an exception happens we say it was thrown or raised When an exception is dealt with, we say the exception is was handled or caught

10. Unchecked Exceptions All the exceptions we've seen so far have been Unchecked Exceptions, or Runtime Exceptions Usually occur because of programming errors, when code is not robust enough to prevent them They are numerous and can be ignored by the programmer

11. Common Unchecked Exceptions NullPointerException reference is null and should not be IllegalArgumentException method argument is improper is some way IllegalStateException method called when class is in improper state

12. Checked Exceptions There are also Checked Exceptions Usually occur because of errors programmer cannot control: examples: hardware failures, unreadable files They are less frequent and they cannot be ignored by the programmer...

13. Dealing With Checked Exceptions Every method must catch (handle) checked exceptions or specify that it may throw them Specify with the throws keyword void readFile(String filename) { try { FileReader reader = new FileReader("myfile.txt"); // read from file... } catch (FileNotFoundException e) { System.out.println("file was not found"); } void readFile(String filename) throws FileNotFoundException { FileReader reader = new FileReader("myfile.txt"); // read from file... } or

14. Exception Class Hierarchy Exception RuntimeException ArrayIndexOutofBounds NullPointerException IllegalArgumentException etc. IOException FileNotFoundException MalformedURLException SocketException etc. SQLException Unchecked Exceptions Checked Exceptions All exceptions are instances of classes that are subclasses of Exception

15. Checked and Unchecked Exceptions Checked ExceptionUnchecked Exception not subclass of RuntimeException subclass of RuntimeException if not caught, method must specify it to be thrown if not caught, method may specify it to be thrown for errors that the programmer cannot directly prevent from occurring for errors that the programmer can directly prevent from occurring, IOException, FileNotFoundException, SocketException NullPointerException, IllegalArgumentException, IllegalStateException

16. Exception Constructors Exceptions have at least two constructors: 1. no arguments NullPointerException e = new NullPointerException(); 2. single String argument descriptive message that appears when exception error message is printed IllegalArgumentExceptione e = new IllegalArgumentException("number must be positive");

17. Writing Your Own Exception To write your own exception, write a subclass of Exception and write both types of constructors public class MyCheckedException extends IOException { public MyCheckedException() {} public MyCheckedException(String m) {super(m);} } public class MyUncheckedException extends RuntimeException { public MyUncheckedException() {} public MyUncheckedException(String m) {super(m);} }

18. Throwing Exceptions Throw exception with the throw keyword public static int average(int[] a) { if (a.length == 0) { throw new IllegalArgumentException("array is empty"); } int total = 0; for(int i = 0; i < a.length; i++) { total += a[i]; } return total / a.length; }

19. Keyword Summary Four new Java keywords try and catch – used to handle exceptions that may be thrown throws – to specify which exceptions a method throws in method declaration throw – to throw an exception

20. Throws and Inheritance A method can throw less exceptions, but not more, than the method it is overriding public class MyClass { public void doSomething() throws IOException, SQLException { // do something here } public class MySubclass extends MyPlay { public void doSomething() throws IOException { // do something here }

21. Line Intersection Example Consider this class Line, which has two fields, a slope and a yIntercept Let's write an intersect method that returns the x-coordinate at which the two lines intersect. sig Line { private double slope; private double yIntercept; double getSlope() { return slope; } double getYIntercept() { return yIntercept; } }

22. Calculating the x-coordinate at which two lines intersect We could translate this directly into the following intersect method: Boring Math Stuff... y = m 1 x + b 1 y = m 2 x + b 2 m 1 x + b 1 = m 2 x + b 2 m 1 x - m 2 x = b 2 - b 1 (m 1 - m 2 )x = b 2 - b 1 x = (b 2 - b 1 )/(m 1 - m 2 ) double intersect(Line line1, Line line2) { return (line2.getYIntercept() – line1.yIntercept()) / (line1.slope() – line2.slope()) }

23. Parallel lines will never intersect. If lines are parallel, then their slopes will be equal, line1.slope() – line2.slope() = 0, and our method will attempt to divide by zero Let's write a new exception ParallelException to throw when this occurs. What About Parallel Lines?

24. ParallelException ParallelException will be a checked exception because calculating whether lines are parallel is not something we expect the programmer to know how to do and prevent in advance. Checked exceptions are a subclass of Exception, but not RuntimeException public class ParallelException extends Exception { public ParallelException() {} public ParallelException(String msg) { super(msg); } }

25. Final Intersect Method Because it is a checked exception, intersect must specify that it throws it double intersect(Line line1, Line line2) throws ParallelException { if (line1.slope() = line2.slope()) { throw new ParallelException(); } return (line2.getYIntercept() – line1.yIntercept()) / (line1.slope() – line2.slope()) }

26. Calling the intersect Method A method that accepts two Lines as arguments, calls intersect, and prints out the results: void printIntersect(Line line1, Line line2) { try { double x = intersect(line1, line2); System.out.println("Intersect at " + x); } catch (ParallelException e) { System.out.println("They are parallel"); }